a. Field of the Invention
This invention relates to an overrunning clutch. In particular, the instant invention relates to a clutch having a structure that enables improved control of clutch engagement, and smaller, more evenly distributed loads on clutch engagement surfaces.
b. Background Art
Clutches are used in many applications to selectively engage and disengage a driving device such as a motor with a driven device (e.g. a conveyor or a reel for a hose or cable) in order to transfer torque from the driving device to the driven device. In many clutches, engagement and/or disengagement occurs through electro-magnetic or fluid actuation or even manually by the user. These actuation methods increase the operating complexity and cost of the clutch. To address this drawback, in other clutches engagement and/or disengagement occurs mechanically through, for example, the use of springs and/or in response to external forces operating on the clutch such as centrifugal force.
One conventional type of clutch that relies on mechanical actuation is a centrifugal clutch. In a centrifugal clutch, a radially inner member of the clutch is coupled to the driving device. When the rotational speed of the driving device reaches a pre-determined level, shoes attached to the inner member of the clutch move radially outward against the bias of springs and engage a radially outer member of the clutch coupled to the driven member. Centrifugal clutches have several disadvantages, however. First, as the shoes engage the surface of the radially outer member, the shoes slide until the centrifugal force reaches a sufficient level to transmit torque. This sliding motion results in friction that increases temperatures within the clutch and wear on the surfaces of the clutch. The wear generates metal particles that abrade the surfaces of the clutch and cause even greater wear. Second, to achieve sufficient centrifugal force, the angular speed and mass of the shoes must be relatively high. As a result, the clutch is relatively large.
Another conventional type of clutch that relies on mechanical actuation is a ratchet and pawl clutch. In this type of clutch, pawls are brought into engagement with grooves formed in the surface of one of the clutch members either by spring loading or by centrifugal force. This type of clutch, however, also has several disadvantages. Formation of the grooves requires specialized manufacturing and heat treatment to increase material hardness thereby increasing manufacturing costs and complexity. The grooves also have sharp corners and edges that act as stress risers and reduce the clutch's strength and durability. Further, the relatively flat surfaces of the groove make it difficult to control the exact position of engagement by the pawls and, when multiple pawls are involved, it is common for one or more pawls to engage before others causing uneven load sharing. Further still, disengagement of the pawls from the grooves requires reverse relative motion between the clutch members to provide sufficient clearance. In ratchet and pawl clutches where clutch engagement results from centrifugal force, there are still further disadvantages. In particular, as rotational speed increases and the pawls move outward, it is difficult to prevent unwanted contact between the sharp edges of the pawls and the grooves and the resulting damage to both. Further, the pawls engage the surfaces of the grooves at relatively high speeds thereby generating high impact loads. To counteract these loads, the clutches tend to be relatively large and made from high strength materials thereby increasing costs.
The inventor herein has recognized a need for an overrunning clutch that will minimize and/or eliminate one or more of the above-identified deficiencies.